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[avr-libc-dev] Re: eeprom_read_byte and clr ret_hi

From: David Brown
Subject: [avr-libc-dev] Re: eeprom_read_byte and clr ret_hi
Date: Mon, 23 Nov 2009 11:24:38 +0100
User-agent: Thunderbird (Windows/20090812)

Weddington, Eric wrote:

-----Original Message----- From: address@hidden [mailto:address@hidden org]
On Behalf Of Dmitry K. Sent: Sunday, November 22, 2009 12:21 AM To:
address@hidden Subject: Re: [avr-libc-dev]
eeprom_read_byte and clr ret_hi

eeprom_read_byte returns a uint8_t. Why does it clear r25? eerd_byte.S: clr ret_hi

Does the AVR ABI require that r25 be zeroed in a function
returning a
single byte? If not, this instruction could be removed.
This is a misty point. Look an example:

unsigned char foo1 (unsigned char *p) { return *p; }

extern unsigned char ext2 (void); int foo2 (void) { return ext2() +
1; }

Old Avr-gcc (3.3 - 4.2) are clear R25 in both cases: foo1() and
foo2().  The new Avr-gcc (4.3.3 and 4.4.2) are not clear R25 in

Note, the function return value is present only in expression. So
it is promoted to integer. So it would be better to clear R25 in
foo1() only (at one place).

I agree that that is the way it should be.

I'm a little confused by this - I hope this is not implying a return to avr-gcc 4.2 R25 clearing? With avr-gcc 4.2, foo1() above would clear R25 even though it is returning an 8-bit value. This has always been a waste of time and space - caller functions make no use of the cleared R25, and thus clear it again themselves (such as in foo2() after calling ext2()). With avr-gcc 4.3, the extra clear R25 instructions are omitted for functions returning 8-bit values. This is the way it should be (unless the C standards disagree...), IMHO. But it looks a little like you want foo1() to clear R25 here?

Incidentally, avr-gcc 4.2.2 actually produces better code for foo2() than avr-gcc 4.3.2.

With 4.2.2, the code is:
        call ext2
        ldi R25, lo8(0)
        adiw r24,1

With 4.3.2 (and 4.3.0), we get:
        call ext2
        mov r18, r24
        ldi r19, lo8(0)
        subi r18, lo8(-(1))
        sbci r19, hi8(-(1))
        movw r24, r18

Is this regression is news to you, I can take it up in the main avr-gcc mailing list and/or a missed optimisation bug report.



Possibly, it is needed to change eeprom_read_byte() definition to
int return value. This can reduce summary size. Opinions?

This does not make sense to me. Eeprom_read_byte() is supposed to
read, and return, a single byte. So why should it return a 16-bit
int? In my mind this would just make it more confusing to the end

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